Spatial characterization of nutrient dynamics in the Bay of Tunis (south-western Mediterranean) using multivariate analyses: Consequences for phyto- and zooplankton distribution

The hydrological structure and nutrient dynamics of the Bay of Tunis (south-western Mediterranean), and the abundance of its principal plankton groups (diatoms, dinoflagellates, tintinids, rotifers, appendicularians, copepods, medusae, siphonophores, chaetognaths and cladocerans), were studied over 2 years. Despite the small size of the Bay (361 km2), the nutrient concentrations varied greatly between the sampling stations. We distinguished spatial from temporal variations using a newly-developed method for regionalization based on multivariate cluster analysis, and the changes over time of any similarities between stations. The method allowed us to obtain an optimal geographical division of any degree of grouping of the sampling stations (i.e. for a chosen number of zones). We also calculated the optimum partition of stations using a multivariate matrix obtained from multi-parameter sampling over time. The application of the method to a time-series of nutrient concentrations in the Bay of Tunis produced four zones of similar nutrient dynamics. Each zone was then characterized by the median and the variability of its nutrient, physical and biological parameters. A statistical test was used to assess the significance of the differences between zones for the various parameters. Comparison of the medians of the various parameters allowed us to synthesize the underlying differences in nutrient dynamics and also in plankton ecosystem components. This analysis revealed a relative nutrient enrichment in the west and south-western areas of the Bay. Silicates did not contribute to the high variability of nutrients in the Bay. The pattern of phytoplankton biomass (chlorophyll a) and the abundance of diatoms and dinoflagellates were similar to those of the sampled nutrients. The highest values were measured in the first zone, where the highest nutrient concentrations were also recorded. The coastal zones were characterized by zooplankton groups of small size (tintinids, rotifers and small copepods) with a limited diversity. These zones were also very turbid and shallow. Central zones, on the other hand, were rich in zooplankton species. Meso-zooplankton groups (e.g. siphonophres, chaetognaths) were also more abundant in this zone.